You'd be unlikely to recognise Julia Davis if you saw her in a bar. I'm in a bar and I don't recognise her. And I've got an appointment to meet her so I'm actively looking. Plus, I spent the previous evening watching her on telly. Trouble is, her appearance changes almost entirely from one role to the next. In the space of three hours I saw her portray everything from a gormless and strikingly unattractive bride-to-be from the Welsh valleys to an uptight yet strangely sexy toff from the shires. Seeing the real Julia Davis, stripped of special wigs and other thespian accoutrements, is a rare experience. Since she first gained high profile success with award-winning BBC2 comedy Human Remains a couple of years back, she has purposefully taken a back seat while her writing and performing partner on the series, Rob Brydon, earned a reputation as one of the country's hottest new comics.

But the year ahead will almost certainly see her wrenched out of the shadows. She's set to star in Nighty Night, a sitcom she wrote for the BBC. Its chances of success will be assisted by the facts that a) it features a roll call of top-notch British comedy actors, from The League Of Gentlemen's Mark Gattiss to My New Best Friend's Marc Wootton to tabloid scandal's Angus Deayton and b) it's really funny. She'll be simultaneously starring in the BBC's adaptation of The Alan Clark Diaries as frosty secretary to John Hurt's Clarke. All of which is likely to officially inaugurate her as the First Lady of British comedy. It's a role that's been vacant since Caroline Aherne took leave of absence, and Davis has all the credentials: she already has a Royal Television Society Award for Human Remains, writes almost all her own material and has worked with every top comedy power in the country, including Brydon, Steve Coogan, Meera Syal, Father Ted creators Graham Linehan and Arthur Matthews, plus Chris Morris.

The only thing that is consistent about the characters Davis plays is that, to varying degrees, they tend to be spiteful, cruel and heartlessly rude. But this doesn't make her more recognisable either because, in person, she's dead nice with pretty features, big friendly eyes and a demeanour that's generally rather shy.

"If I claim I'm the opposite of my characters then it'll just sound awful," she says when she's finally identified herself to me and we've sat down for a cup of tea. "But I tend to write the sort of things I'd never say because I'm not a very forceful person."

Her character in Nighty Night, in contrast, is the sort of woman who reacts to the news that her husband has cancer with the words "Why me?" Davis specialises in portraying dysfunctional relationships in grotesque detail. Human Remains featured a woman who had convinced her husband she had a genital disorder that prevented her from having sex but seduced other men behind his back. In Nighty Night her character tells her husband his cancer is punishment for a one-night stand he had years earlier.

She's responsible for some of the darkest comedy ever seen on British television. "I just find it funny and terrible someone being very rude and overbearing over somebody who doesn't know how to deal with it," she says. "Maybe it's because I've experienced that sort of thing and I don't know how to say, 'You can't do that. You can't say that to me.'"

Obviously, Davis isn't totally devoid of self-confidence. In the mid-1990s, after dropping out of a drama course in York, she was working in a Tesco in her hometown of Bath, surrounded by women 20 years her senior crowing, "You'll never make it out of here Julia."

"I used to tell them, 'I will, I'm going to London,'" she recalls. "But they were like, 'You're not Julia. You'll never get out.'"

At that stage Steve Coogan and Chris Morris were heroes she'd watch on the telly. Within a couple of years she was working with them both. While still at Tesco, she joined a local drama group where she began to develop comedy character ideas with a friend. That material got her involved with an improvisation group where she met Rob Brydon for the first time. Her confidence grew from performing in public and she sent some sketch ideas in to Radio 4, who hired her to work on a female comedy show with Arabella Weir and Meera Syal. She then decided to send some ideas to Coogan who hired her to work on his touring live show.

"I had a feeling he might like my characters when I sent them in," she remembers. "But he probably thinks my stuff's darker than his. I think he thinks I'm quite odd."

When Brydon heard she was working with Coogan he asked her to pass on one of his tapes. Before long, the pair had made Human Remains for Coogan's production company, Baby Cow. Her experiences of working with Brydon and Coogan are vastly different, she says. "When you improvise with Rob it's very natural and just feels like mucking about. With Steve - and this will sound like I'm slagging him off which I'm not - he's much more like: 'Why don't you say this or why don't you say that?'"

Her own obsessive approach startles those she works with. "There's no room for compromise with Julia," says Brydon. "She is a purist. She totally immerses herself in every character she plays. It's both exciting and frightening."

It's Chris Morris, the elusive creator of Brass Eye and The Day Today, whom she holds most respect for. "I love working with Chris and love anything to do with him," she says recalling her time working with him on the surreal Blue Jam and TV follow-up Jam. "He is revered among comedians. He's the one people always say they want to work with. They're all secretly respectful of the fact that he's not caught up with the whole media thing."

It's an approach she's keen to employ herself, although this will be much harder once she starts to dominate schedules in the coming weeks. "I just want to do interesting work and avoid all that personality stuff. I'd hate to go on a chat show or anything like that. The very idea of people wanting to know about your life is awful." Which seems as good a note as any to end the interview and allow her to disappear anonymously from the bar. *